1. Field of the Invention
The present invention concerns a multichannel optical spectrum slicer for outputting discontinuous multichannel lights of different wavelengths or multiple wavelength lights from a broad band continuous spectrum source having a wavelength region of a predetermined range, which is suitable as a light source for inspection and evaluation of optical parts, devices and systems in the wavelength division multiplexer for use in optical communications systems.
2. Statement of the Related Art
In optical communication using optical fibers as signal transmission lines, TDM(time division multiplexing) transmission has been conducted so far with an aim of higher bit-rate transmission of a great amount of information and, recently, WDM(wavelength division multiplexing) transmission has been noted for transmitting a further great amount of information along with popularization of internets.
The WDM transmission is a mode for multiplex transmission of a plurality of light signals of different wavelengths by way of a single optical fiber. Optical signals from light sources of different wavelength are synthesized by an optical multiplexer into a single optical fiber on the transmission side, while an optical signal from the optical fiber is separated on every wavelength by an optical demultiplexer and convert the same into electric signals by a photoreceiving device on the receiving side.
In this constitution, a plurality of signals independent of each other can be transmitted by using a single optical fiber, to provide advantages capable of bilateral transmission, transmitting different kinds of signals such as analog signals and digital signals simultaneously, and transmitting signals at high speed and of large capacity while dividing them into channels each at low speed and of small capacity.
By the way, for conducting the WDM transmission, a light source device for outputting light at a predetermined wave length has been obtained mainly by controlling the wavelength of a laser light source or transmitting a light outputted from an light emission diode through an interference filter to selectively take out a light at a desired wavelength.
However, since any of the light sources described above can output only the light at a single wavelength, when a plurality of lights of different wavelengths are intended to be multiplexed, light sources are required by the number of channels, to increases the cost.
In the WDM transmission, it is desirable to increase the density by setting the wavelength spacing between each of transmission lights to 1 nm or less (typically 50 and 100 GHz by frequency spacing).
However, even when the laser light sources are used by the number corresponding to the number of channels, it is practically difficult to control the wavelength of each laser light at a high accuracy and output discontinuous multiple wavelength lights with a spacing of 1 nm or less between each of adjacent lights.
Further, since the interference filter for use in high DWDM(dense wavelength division multiplexing) transmission has a multi-layered structure of 50 to 100 layers, it is extremely difficult to design and manufacture the filter such that discontinuous multiple wavelength lights can be outputted with the wavelength spacing of 1 nm or less between each of adjacent lights by controlling the thickness for each of the layers.
Furthermore, in the WDM transmission, it is important that spectrums of multiple wavelength lights do not overlap with each other in a state where the lights of different wavelengths are synthesized or separated such that lights of wavelengths adjacent with each other cause no cross-talk.
As another subject, since lights of various wavelengths transmit through optical parts, devices and systems in the WDM optical communication, it is necessary to previously recognize the optical characteristics of them to all of such wavelengths and this may be confirmed by entering discontinuous multiple wavelength lights having a desired wavelength spacing to the optical parts, devices and systems and detecting the characteristic of emission light.
In view of the above, it is a first technical subject of the present invention to provide a multichannel optical spectrum slicer capable of outputting discontinuous multiple wavelength lights having a desired wavelength spacing from a broad band continuous spectral source in a simple structure and at a reduced cost without using special light sources or filters.
A second object of the present invention is to enable easy measurement for modal birefringence and/or polarization mode dispersion of a birefringent device used for such channel optical spectrum slicer.
For solving the subject, the present invention provides a multichannel optical spectrum slicer for converting a light incident from a broad band continuous spectrum light source having an optional wavelength region into discontinuous multiple wavelength lights, and outputting the same, comprising:
a birefringent device such as a birefringent single-mode optical fiber or uniaxial single crystal member having two polarization axes each orthogonal to an optical axis (z) and
linear polarizers disposed at the light incident end and the light emission end of the birefringent device with the direction of polarization being inclined by about 45xc2x0 relative to each of polarization axes.
According to the invention, when a broad band continuous spectral source having an optical wavelength region transmits the linear polarizer on the incident side, it is converted into linearly polarized lights each in 45xc2x0 direction and entered to the birefringent device to form a light polarized along axis x (x-polarized light) and a light polarized along axis y (y-polarized light) each transmitting along the optical axis, in which the light intensity is identical between both of the polarized light components.
Since the refractive indexes nx and ny of the birefringent device are different with respect to the two polarization axes, a difference in the velocity is caused between the x-polarized light and the y-polarized light to form a phase difference at the emission end.
Accordingly, when the lights transmit the linear polarizer on the emission side, 45xc2x0 components of the x-polarized light and the y-polarized light are synthesized, and the identical spectral components interfere with each other, so that a comb type spectrum of the discontinuous multiple wavelength light is observed in the spectral region by wavelength scanning of the emission light by a spectral analyzer or the like.
Modal birefringence xcex94n=nxxe2x88x92ny of the birefringent device can be determined by the following equation, assuming the frequency spacing between adjacent spectrums of the observed discontinuous multiple wavelength lights as vp and the known length of the birefringent device as L0:
xcex94n(v)=c/{vp(v)xc2x7L0}
Further, the polarization mode dispersion PMD of the birefringent device can be determined correctively by using the modal birefringence xcex94n as:                               PMD          =                                    ⅆ              Δβ                        /                          ⅆ              ω                                                                    =                                    Δ              ⁢                              xe2x80x83                            ⁢                                                n                  ⁡                                      (                    v                    )                                                  /                c                                      +                                          (                                  v                  /                  c                                )                            ⁢                              {                                                      ⅆ                    Δ                                    ⁢                                      xe2x80x83                                    ⁢                                                            n                      ⁡                                              (                        v                        )                                                              /                                          ⅆ                      v                                                                      }                                                              Δβ    =          2      ⁢              xe2x80x83            ⁢      π      ⁢              xe2x80x83            ⁢              v        ·        Δ            ⁢              xe2x80x83            ⁢                        n          ⁡                      (            v            )                          /        c                  ω    =          2      ⁢              xe2x80x83            ⁢      π      ⁢              xe2x80x83            ⁢      v      
Then, discontinuous multiple wavelength lights with a predetermined frequency spacing vp can be outputted by using a birefringent device made of the identical material and setting the length L=c/(xcex94nxc2x7vp), based on the thus determined modal birefringence xcex94n.